Conventionally, there has been widely employed a structure in which a catalyst is arranged in the exhaust path of an engine so as to be utilized for purifying the exhaust gas. As a typical structure, there can be listed up a three-way catalyst of a gasoline engine, or a nitrogen oxide occlusion reduction type catalyst arranged integrally with or in a front side of a particulate filter in a diesel engine and promoting oxidization (incineration) of the collected particulate or the like. Some V-type engines and the like employ a structure in which a plurality of exhaust paths are arranged in parallel, for example, an exhaust gas purifying apparatus provided with a catalyst device in each of a pair of exhaust paths arranged in parallel is disclosed in patent documents 1 and 2.
In the apparatus disclosed in patent document 1, a pair of exhaust paths are connected by a connection conduit pipe in an upstream side of the catalyst device, and a throttle valve is provided in a portion of the exhaust path in a downstream side of a joint portion between one of the exhaust paths and the connection conduit pipe and in an upstream side of the catalyst device. In a low temperature state in which the catalyst device is not sufficiently activated, the throttle valve is closed, and the exhaust gas is discharged only from the other exhaust path.
In the apparatus disclosed as a fourth embodiment of patent document 2, a first exhaust passage and a second exhaust passage are connected via a switching valve in an upstream side of the catalyst device. In a high temperature state in which the catalyst device is sufficiently activated, the switching valve can be switched to a state in which the exhaust gas is discharged from both of the exhaust paths.
In a low temperature state (for example, just after the engine start) in which the catalyst device is not sufficiently activated, it is hard to expect the exhaust gas purification by the catalyst device. The structure in which the exhaust gas is discharged only from one exhaust path quickens a temperature rise of the catalyst device arranged in the exhaust path through which the exhaust gas passes so as to promote an early activation of the catalyst device.
However, in the apparatus disclosed in the patent documents 1 and 2, there is required a complicated structure to which a new piping and a new valve apparatus for communicating a pair of exhaust paths in the upstream side of the catalyst device.
Patent Document 1: Japanese Laid-Open Patent Publication No. 50-13718
Patent Document 2: Japanese Laid-Open Patent Publication No. 3-281929
Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-269155